Use of cellular phones as photo-cameras requires increased current use. Enhancement of image quality in scarcely illuminated scenes also requires a powerful LED. Batteries are useful to deliver peak current beyond a certain limit and this leads to the development and use of so-called “super-capacitors”. The capacitance of these components may be on the order of one Farad, and thus are able to accumulate a large amount of energy and to deliver it in a very short time (as required by a photo-camera flash).
A super-capacitor (SC) typically includes two or more capacitors, also called capacitance cells, in series for supplying a voltage larger than that of an available voltage source, for example, a battery. For example, for common FLASH applications that use white-light power LEDs with a threshold voltage higher than 4V, present fabrication technology of silicon integrated circuits may ensure that a single integrated capacitance cell will withstand a voltage of about 2.5V at most without degrading. This may be insufficient.
To address the problem, at least two capacitance cells are connected in series as shown in FIG. 1, obtaining a three-terminal integrated component in case of only two cells capable of withstanding up to 5V on its opposite end nodes, or generally a multi-terminal integrated component when more than two cells are connected in series. The central or any of the intermediate nodes of the series is accessible from outside the integrated circuit device because, when charging the multi-cell integrated capacitor each of the single capacitance cells or capacitor of the series should be charged at the same voltage.
In practice, it may be important that, while charging them, none of the capacitance cells be subjected to a voltage in excess of the maximum threshold of the fabrication technology (e.g. 2.5V in the considered case) and that, at the same time, in each cell be stored the maximum possible energy. However, every integrated capacitance cell generally has a small but non-null leakage current. Therefore, considering the exemplary case of only two cells in series, if they have different leakage currents, the voltage on the central node of the three terminal component shifts higher or lower than the mid-value of the applied charge voltage, with the risk of exceeding the breakdown voltage of the cell with the smallest leakage.
A simple way of controlling the voltage on the central node as shown in FIG. 2 includes using a unity gain operational amplifier suitably connected for regulating its output node to remain always at the half value of the applied charge voltage. The drawbacks of this simple approach are the relatively large consumption and the need for an applied charge voltage source that should desirably not become smaller than the voltage at which the multicell integrated super-capacitor is charged. Moreover, it may be desirable to ensure control so that no excessive inrush currents are produced because of the very low equivalent resistance of the super-capacitor.
A simple and immediate way of charging a super-capacitor and addressing the problem of excessively large inrush currents includes using a voltage regulator connected in series with an output current limiter, as disclosed in the published patent application FR 2,838,572. With this approach, the current supplied by the voltage source (for example a battery) is limited and in addition the charge voltage of the super-capacitor is controlled.
Unfortunately, this approach, for a super-capacitor including two or more capacitors in series, may require a control circuit for the voltage on the central node of the series connected capacitors.